Polling stations open at 8 am (0600 GMT) and close at 8 pm, followed immediately by two exit polls. The focus will be on challenges in the Nordic country's welfare model and on two new far-right anti-immigrant parties.
Police say more than 750 people have been arrested and some 28 have been charged since climate change protests began earlier this week in London.
Some 1.5 million Finns have already cast their ballots during a week of advance voting this month.
In its latest State of the Climate overview, the World Meteorological Organization (WMO) reaffirms that the past four years have been the hottest on record - figures previously announced in provisional drafts of the flagship report.
Twenty warmest years "all in the last 22," scientists say in a UN report, with weather extremes in 2018 including wildfires in California and Greece, drought in South Africa and floods in Kerala, India.
President Jair Bolsonaro who threatened to pull Brazil out of the Paris climate accord during his election campaign pledged at Davos to work "in harmony with the world" to protect his country's unique environment.
Scientists say that it is vital to limit the rise in global temperatures to 2 Celsius to avert more extreme weather, rising sea levels and the loss of plant and animal species, although limiting the rise to 1.5C would have a far greater benefit.
A meeting of 195-nation Intergovernmental Panel on Climate Change in South Korea is vetting a major report that traces pathways for cutting global warming to 1.5 degrees Celsius. The bombshell 20-page report is due to be released on October 8.
Farmers in Australia's eastern states are struggling with extreme aridity that has turned green pastures into dust, with the economies of local towns also suffering.
While talking to local media, provincial health secretary rejects claims that any one has died from heat-stroke in Pakistan's largest city.
Some residents on Plum Island in Massachusetts are facing the threat of losing their homes due to climate change.
Up to 25,000 people are expected to attend the UN's annual conference on fighting climate change in Germany.
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